%<name|builtin>
%!<name|builtin> # doesn't do anything but is allowed
%?<name|builtin> # defined expression
%!?<name|builtin> # negated defined expression

%{<name|builtin>}
%{<name>: ...} # What is the ':' for an operator here? Is it only valid for builtins?
%{?<condition>:<consequence>} # Ternary expression? Normally ternary expressions are <condition> ? <consequence> : <alternative>
%{!?<condition>:<consequence>} # Negated ternary expression

%[...] # I have no idea what this does, first time I stumbled upon it.

%(...) # Macro expansion allowed inside